1. Field of the Invention
This invention relates to a zoom lens, and in particular to a zoom lens of high performance in which fluctuation of aberrations is small during magnification change.
2. Description of the Prior Art
Generally, a zoom lens has it focal length variable continuously and can therefore vary its image magnification continuously for an object at a predetermined distance and thus is very convenient, but in the zoom lens, the fluctuation of aberrations resulting from magnification change is great and therefore it is difficult to maintain an excellent imaging performance over the entire magnification change range. Accordingly, to change the photographing magnification in the enlargement work of photographed film or the reduction (enlargement) copying work, the following two methods using a fixed focal length lens have often been adopted.
One of the two methods is a method using a fixed focal length lens to change the object-image distance and the position of the lens, and the other is a method interchangeably using a plurality of fixed focal length lenses to obtain different kinds of magnifications equal to the number of the lenses. The former has a disadvantage that the apparatus becomes bulky because the object-image distance is greatly varied, and the latter has a disadvantage that only dispersive magnifications can be obtained.
To eliminate these disadvantages, in recent years, it has been desired to use a zoom lens also for enlargement or copying and make the most of the above-described advantages of the zoom lens. However, the performance required of the enlarging or copying lens system is very severe and it is necessary to well correct not only distortion and curvature of image field but also lateral chromatic aberration, and it has been very difficult to realize it.